a. Field of the Invention
The instant invention relates to a system and method for measuring an impedance using an ablation catheter.
b. Background Art
Electrophysiology (EP) catheters have been used for an ever-growing number of procedures. For example, catheters have been used for diagnostic, therapeutic, mapping and ablative procedures, to name just a few examples. Typically, a catheter is manipulated through the patient's vasculature and to the intended site, for example, a site within the patient's heart, and carries one or more electrodes, which may be used for mapping, ablation, diagnosis, or other treatments.
There are a number of methods used for ablation of desired areas, including, for example, radio frequency (RF) ablation. RF ablation is accomplished by transmission of radio frequency energy to a desired target area through an electrode assembly to ablate tissue at the target site. RF ablation may generate excessive heat if not controlled. It is therefore known to provide an ablation generator with certain feedback features, such as temperature and impedance. To provide such feedback for the physician/clinician's use during the procedure, conventional RF ablation generators are typically configured to measure and display a magnitude of a complex impedance (Z) at least intended to represent the impedance of the patient's tissue proximate the ablation electrode. To make the impedance measurement, conventional generators use one tip conductor (i.e., one lead through the catheter to the ablation tip electrode) and one RF indifferent/dispersive return (i.e., one lead from the RF indifferent return going back to the generator)—a two terminal configuration for measurement. The frequency of the source used to make the impedance measurement is generally the ablation energy source frequency, which typically may be around 450 kHz or higher depending on the ablation generator. Such impedance measurements are commonly used to assess tissue heating and tissue-electrode contact. However, one shortcoming in the art is that such two-terminal measurements are subject to variation in the measurement of impedance due to factors unrelated to the condition of the tissue (i.e., non-physiologic changes). For example, coiling of an ablation cable that connects the ablation generator to the catheter can alter the impedance measurement, providing an inaccurate reading that is not completely indicative of the actual tissue condition.
Other impedance measurement techniques are known generally. For example, four-terminal measurements have been considered for the measurement of bulk bioelectrical impedance and measurement of lesions in arteries. However, these other measurement techniques do not address the problems described above.
There is therefore a need to minimize or eliminate one or more of the problems set forth above.